Mobile communication system mobile communication method base station and mobile station

ABSTRACT

An effective random access control can be performed with a high throughput. A base station ( 11 ) communicates with mobile stations ( 12 ) by use of slotted ALOHA system. An offset time of 1.5 frames is established between upstream and downstream communication frames. If any of the upstream communication frames is available, the base station ( 11 ) notifies a transmission permission signal (I) by use of the corresponding downstream frame. If having data to be transmitted, a mobile station ( 12 ) in a reception state receives the transmission permission signal (I), switches itself into a transmission state, and transmits one frame of leading data by use of the upstream frame. If permitting continuous transmission of data following the leading data, the base station ( 11 ) transmits a continuous transmission permission information (P) to that mobile station ( 12 ), and transmits a transmission inhibition signal (B) to the other mobile stations ( 12 ) during that continuous transmission. When receiving the continuous transmission permission information (P), that mobile station ( 12 ) places itself in a transmission mode, and transmits the following data by use of a plurality of continuous frames of the upstream communication frames.

TECHNICAL FIELD

The present invention relates to a mobile communication system, a mobilecommunication method, a base station and a mobile station for executionof random access control.

BACKGROUND ART

Various methods of slotted-ALOHA random access control on mobile unitshave been studied. For example, in a TDMA (time division multipleaccess) method, Idle-Single Casting Multiple Access with Partial Echo(ICMA-PE) in accordance with a digital car phone system standard (ARIBSTD-27) or the like is being used. ICMA-PE itself is described in detailin Non-Patent Document 1.

Under random access control in TDMA such as PDC (Personal DigitalCellular) in Japan, a mobile station alternately performs transmissionand reception according to the characteristics of TDMA. After datatransmission by the mobile station, a base station can enable acondition as to whether or not transmission has been correctly performedby the mobile station to be reflected in reception subsequentlyperformed by the base station, because a sufficient time period fordoing so exists before the time at which reception is to be subsequentlyperformed by the mobile station. Therefore no time slot loss occurs withrespect to time. Also, the mobile station can perform transmission byfull-duplex and therefore can be informed of the results of transmissionto the base station by the next reception.

The base station may be configured so that if the size of datatransmitted from the mobile station is so large that the time requiredfor transmission extends over a plurality of slots, it determines thenumber of remaining slots from information contained in data in theleading slot, inhibits transmission by any other mobile station untilthe number of remaining slots becomes zero, and thereby makes a“reservation” to permit the mobile station that has transmitted theleading slot to perform transmission with priority.

In slotted-ALOHA random access control in an FDMA (frequency divisionmultiple access) method, transmitted and received frames appearcontinuously with respect to time. A new concept is therefore requiredfor notification (transmission), before a time for the next reception bya mobile station, of information as to whether or not a base station hascorrectly received data transmitted by the mobile station.

In systems under a strong demand for a reduction in price, e.g.,wireless communication systems for business purposes, mobile stationsare ordinary configured for half-duplex. A half-duplex-type mobilestation cannot perform transmission during transmission and immediatelyafter the completion of transmission due to switching operation betweentransmission and reception. Therefore, when a base station notifies sucha mobile station about whether or not the base station has correctlyreceived data in the leading frame that the mobile station hastransmitted, it must transmit data indicating the reception result aftera lapse of a certain time period from transmission of the leading frameby the mobile station.

Random access in FDMA is described in a narrow-band digitalcommunication standard (ARIB STD-T61) for example. Also, a controlmethod using the concept of reservation is described in Patent Document1.

An example of operations in accordance with ARIB STD-T61 will first bedescribed.

In ARIB STD-T61, a downstream frame from a base station to a mobilestation contains information for collision control. This collisioncontrol information has the following contents:

-   1) I/B information indicating whether or not the next upstream time    is available (with respect to the same frame number);-   2) R/N information indicating whether or not an upstream signal    having the third preceding frame number has been received; and-   3) PE information indicating a partial echo of the upstream signal    having the third preceding frame number.

In ARIB STD-T61, a mobile station can transmit three preceding frameswithout reception confirmation.

FIG. 11 shows an example of operations in a case where mobile stationsMA and MB each transmit four consecutive frames at times generallycoinciding with each other to cause collision therebetween. It isassumed that in this operation example the radio wave environment forthe mobile station MA is better and information transmitted from themobile station MB does not reach a base station. The operation in FIG.11 will be described below.

When the mobile station MA receives the first downstream frame #1 fromthe base station, it determines that I/B (transmissionpermission/inhibition information) is I (permission) and startstransmission. An information length of 4 frames is recorded in datatransmitted by the mobile station MA. On the other hand, when the mobilestation MB receives the second downstream frame #2 from the basestation, it determines that I/B therein is I and starts transmission. Aninformation length of 4 frames is also recorded in transmitted data.

The base station receives the data transmitted from the mobile stationMA and changes I/B to B in the fourth frame #4 since the transmitteddata is consecutive data (having an information length larger than 1 andcontained in a plurality of frames).

When the mobile station MA receives the fourth frame #4, it determinesthat R/N is R and that a CRC sent in the leading frame from itself andthe received PE coincide with each other. It then determines thatconsecutive transmission can be continued, transmits the final frame #4,and stops transmitting since all the data has been transmitted.

On the other hand, the mobile station MB performs transmission withoutconfirmation until it completes transmission of the third frame #3.However, the transmitted data does not reach the base station because ofcollision with transmission by the mobile station MA. When the mobilestation MB receives a downstream frame (the fifth frame #5 in theexample) after the completion of transmission of the third frame #3, itdetermines that the PE in the downstream frame does not coincide withthe CRC sent in the leading frame from itself. At this point in time,the mobile station MB determines that the data transmitted from itselfhas not reached the base station, i.e., transmission failure, and triesto retransmit after random delay without transmitting the fourth frame.

A random access control method disclosed in Japanese Patent ApplicationLaid-Open No. 2001-285928 will next be described with reference to FIG.12.

In this random access control method, information for collision controlis set together with other data in a downstream frame from a basestation to a mobile station. The collision control information has thefollowing contents:

-   1) I/B information designating permission/inhibition of transmission    by a mobile station at an upstream time (with respect to the same    frame number);-   2) R/N information indicating whether or not an upstream signal    having the third preceding frame number has been received; and-   3) Mobile station information indicating from which mobile station    the upstream signal having the third preceding frame number has been    received.

According to an embodiment described in Japanese Patent ApplicationLaid-Open No. 2001-285928, a mobile station is provided with I/Binformation in three-bit form. Details of this I/B information are asdescribed below.

-   000: Transmission inhibition 1 (“Inhibition 1” in FIG. 8 of this    publication) . . . There is only one remaining upstream transmission    frame signal from a mobile station given a transmission right.-   001: Transmission inhibition 2 (“Inhibition 2” in FIG. 8) . . . The    final upstream transmission frame has been received or an error has    occurred in reception of the upstream transmission frame signal.-   010: Transmission inhibition 3 (“Inhibition 3” in FIG. 8) . . . An    upstream transmission frame including a transmission request has    been received from a mobile station.-   100: Transmission right giving (“Giving” in FIG. 8) Giving a    transmission right to a mobile station which has transmitted an    upstream transmission frame signal of a transmission request without    collision or error-   101: Designated mobile station transmission permission (“Permission”    in FIG. 8) . . . A state in which a particular mobile station is    given a transmission right.-   111: Available (“Available” in FIG. 8) . . . A state in which    transmission of an upstream transmission frame signal from any    mobile station is accepted.

FIG. 12 shows an example of transmission and reception of upstream anddownstream frames in a case where mobile stations MA and MB eachtransmit four consecutive frames.

Operations for this transmission and reception will be described belowby way of example with reference to FIG. 12.

When the mobile station MA receives a downstream frame (assumed to befirst downstream frame #1) from a base station, it determines that I/Bis “Available” and starts transmission. An information length of 4frames is recorded in transmitted data.

The base station detects the signal by a time T1 at which the mobilestation MA starts transmission, and makes I/B “Transmission inhibition3” although reception of the leading frame is not completed. Afterreceiving the entire leading transmission frame from the mobile stationMA, the base station determines whether or not the data transmitted fromthe mobile station MA is consecutive data. In this example, since theinformation length is larger than 1 and since the transmitted data isconsecutive data, the base station gives a transmission right to themobile station MA at a time T2 and makes I/B “Transmission right giving”and makes the mobile station information designative of the mobilestation MA. When the mobile station MA receives the third frame #3including these items of information from the base station, itdetermines that transmission can be continued and transmits all theremaining frames. The mobile station MA thereafter stops transmittingsince all the transmission frames have been transmitted.

On the other hand, at about the time for the second downstream frame #2,the mobile station MB has data to be transmitted. However, the mobilestation MB performs a waiting operation since I/B in the downstreamframe is “Transmission inhibition 3”. At a time T3 after the completionof transmission by the mobile station MA, the mobile station MBdetermines that I/B has become “Available” and starts transmission.

Non-Patent Document 1: Transactions of The Institute of Electronics,Information and Communication Engineers, vol. J76-B-II, No. 3, pp.157-165 “Idle-signal casting multiple access with partial echo(ICMA-PE)”

Patent Document 1: Japanese Patent Application Laid-Open No. 2001-285928

The conventional slotted-ALOHA random access control is designed with aTDMA method in mind. However, it is difficult to perform theconventional slotted-ALOHA random access control as control forprocessing in a base station the contents of data transmitted from amobile station and enabling reflection of a processing result in thenext transmission from the mobile station because transmission andreception are continuously performed with respect to time in FDMA.

In a system in accordance with ARIB STD-T61, a mobile station cantransmit three frames without reception confirmation by a base station.In this system, therefore, a mobile station can start transmissionbefore another mobile station performing transmission completestransmission of leading three frame, so that collision occurs frequentlybetween upstream frames.

In many cases of actual random access transmission from mobile stations,the number of frames is three or less. In transmission of three framesor less, I/B of the downstream frames from a base station is not changedfrom I. As a matter of fact, the chances of control by idle-signalcasting are limited.

Random access control disclosed in patent document 1 has some effect insolving these problems. In random access control disclosed in patentdocument 1, no collision occurs between a particular mobile station andother mobile stations when the particular mobile station is continuouslyperforming transmission with a transmission right given by a basestation, but each and every mobile station is kept in a transmissioninhibited state before given a transmission right. This control methodtherefore has a problem that the throughput is low. Also, this controlmethod requires detecting a transmitted signal when a mobile stationstarts transmitting a leading frame, immediately providing collisioncontrol information reflecting a result of this detection, andimmediately providing collision control information reflecting thecompletion of reception of one transmission frame by a base stationafter the completion of transmission of this transmission frame.

Thus, the load on a base station is large and there are difficulties inproviding a base station configuration and control.

In the control method in accordance with ARIB STD-T61 and the controlmethod disclosed in patent document 1, it is necessary for a mobilestation to perform transmission/reception control in a full-duplexmethod. However, mobile stations such as wireless communication devicesfor business purposes under a strong demand for a reduction in price areordinary configured for half-duplex. It is necessary to perform timingcontrol in such half-duplex mobile stations by considering atransmission-reception switching time. Therefore the above-describedcontrol methods cannot be applied to such mobile stations.

Thus, the efficiency of the conventional random access control is low.There is a demand for random access control of higher efficiency.

The present invention has been achieved in consideration of theabove-described problems of the conventional art, and an object of thepresent invention is to make possible random access control of highefficiency and high throughput.

Another object of the present invention is to make possible randomaccess control with a reduced control load.

Still another object of the present invention is to make possible randomaccess control applicable to a mobile station of a half-duplexconfiguration.

SUMMARY OF THE INVENTION

To achieve the above-described objects, according to a first aspect ofthe present invention, there is provided a mobile communication systemin which a base station (11) and mobile stations (12) performcommunication by a slotted-ALOHA method, the system being characterizedin that a predetermined offset time is set between downstreamcommunication frames from the base station to each mobile station andupstream communication frames from the mobile station to the basestation;

the base station transmits a transmission permission signal (I) forpermitting transmission of one frame of data when a particular one ofthe upstream communication frames is available, determines whether ornot continuous transmission of subsequent data over a plurality offrames should be permitted if the subsequent data exists subsequently tothe one frame of data received through the particular frame from one ofthe mobile stations, and transmits a continuous transmission permissionsignal (P) for permission of the continuous transmission when thecontinuous transmission is permitted; and

if each mobile station has data to be transmitted, it transmits oneframe of data in response to the transmission permission signal (I) fromthe base station, and transmits the subsequent data through a pluralityof consecutive frames in the upstream communication frames whenreceiving the continuous transmission permission signal (P) with respectto the transmitted one frame of data.

For example, the upstream communication frames from the mobile stationto the base station are delayed from the downstream communication framesfrom the base station to the mobile station by a time period longer thanone frame and shorter than two frames; the base station (11) transmitsthe transmission permission signal (I) through the frame in thedownstream communication frames corresponding to the first frame in theupstream communication frames if the first frame in the upstreamcommunication frames is available; if the mobile station (12) has datato be transmitted, it transmits the one frame of data through the firstframe in the upstream communication frames when receiving thetransmission permission signal through the frame corresponding to thefirst frame; determination is made as to whether or not continuoustransmission of subsequent data over a plurality of frames should bepermitted if the subsequent data exists subsequently to the one frame ofdata received through the particular frame from the mobile station, andthe continuous transmission permission signal (P) is transmitted throughthe second frame three frames after the frame corresponding to the firstframe when the continuous transmission is permitted; and when the mobilestation receives the continuous transmission permission signal (P)through the second frame in the downstream communication frames, ittransmits the subsequent data through a plurality of consecutive framesin the upstream communication frames the leading one of whichcorresponds to the second frame.

For example, when the mobile station (12) receives the continuoustransmission permission signal (P), it transmits the subsequent datathrough the consecutive frames without checking whether or not the basestation has succeeded in reception.

For example, the mobile station has a half-duplex-type configurationcapable of selectively executing transmission processing and receptionprocessing; and, when the mobile station receives the transmissionpermission signal in a receiving mode, and if data to be transmittedexists, it transmits the one frame of data in the upstream communicationframe by selecting a transmitting mode in place of the receiving mode,thereafter receives the downstream communication frame by selecting thereceiving mode, and, when receiving the continuous transmissionpermission signal, continuously transmits the subsequent data throughthe plurality of frames in the upstream communication frames byselecting the transmitting mode.

For example, the base station transmits, together with the continuoustransmission permission signal (P), information (R/N, CRC) as to whetheror not one frame of data has been normally received from the mobilestation; and the mobile station determines whether or not the basestation has normally received the one frame of data transmitted from themobile station, and transmits the subsequent data if it determines thatthe base station normally receives the data.

For example, the base station transmits mobile station identificationinformation for identification of one of the mobile stations togetherwith the continuous transmission permission signal (P); and the mobilestation transmits the subsequent data when the mobile stationidentification information designates the mobile station.

For example, the mobile station (12) transmits information foridentification of the number of frames of the subsequent data togetherwith the one frame of data; the base station (11) transmits atransmission inhibition signal (B) for inhibiting data transmission fromthe other mobile stations during transmission of the subsequent data bythe mobile station on the basis of the number of frames of thesubsequent data notified from the mobile station; and the mobile stationthat has transmitted the one frame of data in the mobile stationscontinuously transmits the subsequent data according to the continuoustransmission permission signal, the other mobile stations restrainingthemselves in response to the transmission inhibition signal fromperforming data transmission during transmission of the subsequent data.

For example, the base station transmits a continuous transmissionnon-permission signal (D) when it does not permit continuoustransmission of data subsequent to the one frame of data; and eachmobile station determines whether or not the base station has receivedthe one frame of data transmitted from the mobile station, keeps onstandby for transmission of the subsequent data after a lapse of apredetermined frame period if it determines that the base station hasreceived the data, and if it has received the continuous transmissionnon-permission signal (D), and transmits the leading one frame of datain the subsequent data if it receives the transmission permission signal(I) during standby.

For example, if the mobile station (12) cannot receive the transmissionpermission signal (I) during a predetermined time period in a case whereit has data to be transmitted, it recognizes transmission failure, setsa delay time, and again executes processing for awaiting reception ofthe transmission permission signal after a lapse of time through thedelay time.

For example, the base station includes means of determining whether ornot the continuous transmission should be permitted on the basis of acondition of traffic.

To achieve the above-described objects, according to a second aspect ofthe present invention, there is provided a base station for use in amobile communication system in which communication is performed by aslotted-ALOHA method, the base station being characterized in that apredetermined offset time is set between downstream communication framesfrom the base station to a mobile station and upstream communicationframes from the mobile station to the base station, the base stationhaving:

transmission permission signal transmitting means of transmitting,through a predetermined frame in the downstream communication frames, atransmission permission signal (I) for permitting transmission of oneframe of data when one of the upstream communication frames isavailable;

receiving means of receiving one frame of data transmitted from themobile station through one of the upstream communication framescorresponding to the predetermined frame; and

continuous transmission permission signal transmitting means ofdetermining whether or not continuous transmission of subsequent two ormore frames of data should be permitted if the subsequent frames of dataexists subsequently to the received one frame of data, and transmittingthrough the downstream communication frame a continuous transmissionpermission information (P) for permission of the continuous transmissionwhen the continuous transmission is permitted.

To achieve the above-described objects, according to a third aspect ofthe present invention, there is provided a mobile station for use in amobile communication system in which communication is performed by aslotted-ALOHA method, the mobile station being characterized in that apredetermined offset time is set between downstream communication framesfrom a base station to the mobile station and upstream communicationframes from the mobile station to the base station, the mobile stationhaving:

transmission permission signal receiving means of receiving atransmission permission signal (I) from the base station;

leading data transmitting means of transmitting one frame of leadingdata through one of the upstream communication frames in response to thetransmission permission signal if the data to be transmitted exists;

continuous transmission permission signal receiving means of receivingone of the downstream communication frames subsequently to transmissionof the leading data to receive a continuous transmission permissionsignal; and

continuous transmission means of continuously transmitting subsequentdata subsequent to the leading data through a plurality of frames in theupstream communication frames in response to the continuous transmissionpermission signal.

To achieve the above-described objects, according to a fourth aspect ofthe present invention, there is provided a mobile communication methodin which a first and second communication devices perform communicationby a slotted-ALOHA method, the method being characterized in that apredetermined offset time is set between first communication frames fromthe first communication device to the second communication device andsecond communication frames from the second communication device to thefirst communication device, and a transmission permission signal (I) forpermitting transmission of one frame of data when a particular one ofthe second communication frames is available is transmitted from thefirst communication device to the second communication device;

one frame of data in three or more frames of data to be transmitted istransmitted from the second communication device to the firstcommunication device in response to the transmission permission signal(I);

determination is made on the basis of the one frame of data as towhether or not continuous transmission of data subsequent to the oneframe of data over a plurality of frames should be permitted;

a continuous transmission permission information (P) for permitting thecontinuous transmission is transmitted from the first communicationdevice to the second communication device when the continuoustransmission is permitted; and

the subsequent data is transmitted through a plurality of consecutiveframes in the second communication frames in response to the continuoustransmission permission signal (P).

To achieve the above-described objects, according to a fifth aspect ofthe present invention, there is provided a medium on which a computerprogram is recorded, the computer program enabling a computer having acommunication function to function as a base station for use in a mobilecommunication system in which an offset time is set between downstreamcommunication frames from the base station to a mobile station andupstream communication frames from the mobile station to the basestation, and in which communication is performed by a slotted-ALOHAmethod, the base station having:

transmission permission signal transmitting means of transmitting,through a predetermined frame in the downstream communication frames, atransmission permission signal (I) for permitting transmission of oneframe of data when one of the upstream communication frames isavailable;

receiving means of receiving one frame of data transmitted from themobile station through one of the upstream communication framescorresponding to the predetermined frame; and

continuous transmission permission signal transmitting means ofdetermining whether or not continuous transmission of subsequent two ormore frames of data should be permitted if the subsequent frames of dataexists subsequently to the received one frame of data, and transmittingthrough the downstream communication frame a continuous transmissionpermission information (P) for permission of the continuous transmissionwhen the continuous transmission is permitted.

To achieve the above-described objects, according to a sixth aspect ofthe present invention, there is provided a medium on which a computerprogram is recorded, the computer program enabling a computer having acommunication function to function as a mobile station for use in amobile communication system in which an offset time is set betweendownstream communication frames from a base station to the mobilestation and upstream communication frames from the mobile station to thebase station, and in which communication is performed by a slotted-ALOHAmethod, the mobile station having:

transmission permission signal receiving means of receiving atransmission permission signal (I) from the base station;

leading data transmitting means of transmitting one frame of leadingdata through one of the upstream communication frames in response to thetransmission permission signal if the data to be transmitted exists;

continuous transmission permission signal receiving means of receivingone of the downstream communication frames subsequently to transmissionof the leading data to receive a continuous transmission permissionsignal; and

continuous transmission means of continuously transmitting subsequentdata subsequent to the leading data through a plurality of frames in theupstream communication frames in response to the continuous transmissionpermission signal.

Advantages of the Invention

According to the present invention, random access control of highefficiency and high throughput can be achieved.

According to the present invention, random access control with a reducedcontrol load can also be achieved.

The present invention can also be applied to a mobile station of ahalf-duplex configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of a mobile communicationsystem according to an embodiment of the present invention;

FIG. 2 is a block diagram showing an example of a configuration of abase station shown in FIG. 1;

FIG. 3 is a block diagram showing an example of a configuration of amobile station shown in FIG. 1;

FIG. 4 is a diagram showing the constructions of upstream and downstreamframes formed by the mobile communication system shown in FIG. 1;

FIG. 5 is a flowchart for explaining the basic operation of the basestation;

FIG. 6 is a flowchart for explaining the basic operation of the mobilestation;

FIG. 7 is a timing chart for explaining an example of the operation ofthe mobile communication system;

FIG. 8 is a timing chart for explaining an example of the operation ofthe mobile communication system;

FIG. 9 is a timing chart for explaining an example of the operation ofthe mobile communication system;

FIG. 10 is a flowchart for explaining an example of application of theoperation of the base station;

FIG. 11 is a timing chart for explaining an example of the operation ofa conventional mobile communication system; and

FIG. 12 is a timing chart for explaining an example of the operation ofa conventional mobile communication system.

DETAILED DESCRIPTION OF INVENTION

Random access control according to an embodiment of the presentinvention will be described with reference to the drawings with respectto an example of a mobile wireless communication system to which thisrandom access control is applied.

The mobile wireless communication system to which random access controlaccording to this embodiment is applied is constituted by, as shown inFIG. 1, a base station 11 and a plurality of mobile stations 12 existingin a communication area around the base station 11.

The base station 11 is connected to other base stations 11 through anetwork. The base station 11 and each mobile station 12 perform variouscontrol operations relating to ordinary portable telephonecommunication. However, the following description is made mainly ofportions relating to random access control.

The base station 11 is an apparatus supporting a full-duplexcommunication method in an FDM (frequency division multiplexing) method.As shown in FIG. 2, the base station 11 has a control section 101, astorage section 102, a receiving section 102, a transmitting section 104and an antenna 105.

The control section 101 is constituted by a CPU (central processingunit) and other components. The control section 101 performscommunication control by executing an operation program stored in thestorage section 102.

The control section 101 has, as components relating to random access, adata transmitting/receiving section 111, a header information analysissection 112, a collision control information generation section 113, aCRC (cyclic redundancy check) computation section 114 and a reservationcounter 115.

The data transmitting/receiving section 111 controls datatransmission/reception between the base station and the mobile stations12.

The header information analysis section 112 analyzes header informationset in a received frame (data transmitted on a frame-by-frame basis frommobile station 12) and determines whether or not the frame is a leadingframe and the total number of frames to be transmitted (data length) forexample.

The collision control information generation section 113 generatescollision control information to be transmitted to all the mobilestations 12 in the communication area to control the mobile stations 12so that the mobile stations 12 do not simultaneously transmit datacontinuously. The collision control information will be described indetail.

The CRC computation section 114 performs processing for checking a CRCcode in received data for example.

The reservation counter 115 is a counter for counting the number ofreceived frames of remaining data when a plurality of frames of data aresuccessively received from a particular one of the mobile stations 12.

The storage section 102 stores the operation program, fixed data and thelike for the control section 101.

The receiving section 103 receives data from each mobile station 12through the antenna 105, performs demodulation for restoring the data,for example, by converting the data into a base band signal, andprovides the data to the control section 101.

The transmitting section 104 receives from the control section 101 data(base band signal) to be transmitted to the mobile stations 12,modulates a signal to be transmitted with this data, amplifies thesignal and transmits the signal through the antenna 105.

Each mobile station 12 is a device supporting a full-duplexcommunication method in an FDMA (frequency division multiplex access)method. As shown in FIG. 3, each mobile station 12 has a control section201, a storage section 202, a receiving section 203, a transmittingsection 204 and an antenna 205.

The control section 201 is constituted by a CPU (central processingunit), a DSP (digital signal processor) and other components. Thecontrol section 201 performs communication control by executing anoperation program stored in the storage section 202.

The control section 201 has, as components relating to random access, adata transmitting/receiving section 211, a header information analysissection 212, a transmission start wait timer 213, a random delay section214, a reservation wait counter 215 and a recycle counter 216.

The data transmitting/receiving section 211 controls datatransmission/reception between the mobile station and the base station11.

The header information analysis section 212 analyzes header informationset in a received frame (data transmitted on a frame-by-frame basis frombase station 11) and obtains, for example, collision control informationcontained in the header information.

The transmission start wait timer 23 counts time elapsed after start oftrying data transmission by the control section 201.

The random delay section 214 has a random number generation function fordetermining a random delay time and a timer for measuring the time(delay time) corresponding to a generated random number. When an eventoccurs in which after transmission of a leading frame by the mobilestation 12 the base station 11 cannot suitably receive this frame, therandom delay section 214 determines a time to retransmit the leadingframe.

The reservation wait counter 215 counts, in a case where data to betransmitted is a plurality of frames, the number of frames until a“reservation” for continuous transmission of remaining data is madeafter transmitting the leading frame and after the base station 11 hassuitably received the leading frame.

The recycle counter 216 counts the number of successive occurrences ofan event in which after transmission of a leading frame by the mobilestation 12 the base station 11 cannot suitably receive the frame.

The storage section 202 stores the operation program, fixed data and thelike for the control section 201.

The receiving section 203 receives data from the base station 11 throughthe antenna 205, performs demodulation for restoring the data, forexample, by converting the data into a base band signal, and providesthe data to the control section 201.

The transmitting section 204 receives from the control section 201 data(base band signal) to be transmitted to the base stations 11, modulatesa signal to be transmitted with this data, amplifies the signal andtransmits the signal through the antenna 205.

The random access method executed between the thus-arranged base station11 and mobile stations 12 is assumed to be a slotted-ALOHA method basedon the ICMA-PE method in an FDM/FDMA system taking a frame constructionsuch as shown in FIG. 4.

That is, a transmitting-receiving offset time corresponding generally toa 1.5-frame period is provided between a downstream signal (downstreamframe) and an upstream signal (upstream frame), and frame numbers arerespectively assigned to the frames.

The mobile station 12 is of a half-duplex communication type incapableof performing reception when performing transmission, and incapable ofperforming transmission when performing reception. Therefore the mobilestation 12 performs (sets) a receiving operation (receiving mode) and atransmitting operation (transmitting mode) at different times, betweenwhich a switching time is inserted, and is switch-controlled so that theperiod for the receiving operation includes some of the frame periodsfor downstream frames, and the transmitting mode period includes some ofthe frame periods for upstream frames.

A signal transmitted from the base station 11 to the mobile station 12(downstream frame) includes information for collision control as well asother data items. The collision control information has the followingcontents:

-   1) I/B information indicating whether or not an upstream time is    available (with respect to the same frame number);-   2) R/N information indicating whether or not an upstream signal    having the third preceding frame number has been received;-   3) P/D information indicating whether or not continuous transmission    from the next upstream time (with respect to the same frame number)    is permitted; and-   4) PE information indicating a partial echo of the upstream signal    having the third preceding frame number.

Operations in a case where data transmission/reception is performedbetween the mobile stations 12 and the base station 11 by using such aframe construction will now be described.

The operation of the base station 11 will be described with reference tothe flowchart of FIG. 5.

The base station 11 repeatingly executes processing shown in FIG. 5.

In the base station 11, the receiving section 102 receives datatransmitted from one of the mobile stations 12 through the antenna 105,restores the data by demodulation, and supplies the data to the controlsection 101. The control section 101 CRC checks in the CRC computationsection 114 to determine whether or not the received data has beencorrectly received (step S11).

If any data has been received and if the data has been correctlyreceived (step S11; Yes), the control section 101 sets R/N in data to betransmitted in the next downstream frame (the (n+3)th frame if thereceived data is the nth frame) to R indicating that the data has beenreceived, and sets PE in the data to the CRC computed value of thereceived data (step S12).

If the control section 101 determines in step S11 that no data has beenreceived or data has not been correctly received (step S11; No), it setsRN in the data to be transmitted in the next downstream frame to Nindicating that no data has been received, and sets PE in the data to“0” (step S13).

Subsequently to step S12 or S13, the control section 101 determines, bymeans of the header information analysis section 112, from headerinformation in the received data, whether or not the received frame is aleading frame and whether or not there is any remaining frame (stepS14).

If the control section 101 determines that the received data is not aleading frame or that there is no remaining frame (step S14; No), itsets P/D in the data to be transmitted in the next downstream frame to Dfor non-permission of successive reception (continuous transmissionnon-permission signal) (step S15).

If the control section 101 determines in step S14 that the received datais a leading frame and there is one or more remaining frames (step S14;Yes), it determines, from the value of the reservation counter 115indicating the number of remaining frames, whether or not a reservationhas been set for data transmission by one of the mobile stations usingthe next upstream frame (step S16). If there is a reservation (step S16;Yes), the control section 101 advances the process to theabove-mentioned step S15 for reception of the reserved transmission. Ifthere is no reservation (step S16; No), the control section 101 sets P/Din the data to be transmitted in the next downstream frame to P forpermission of continuous transmission (step S17) and sets the number ofremaining frames (the total number of transmission frames obtained bythe header information analysis section 112-1) in the reservationcounter 115 (step S18).

Subsequently to step S16 or S18, the control section 101 againdetermines whether or not there is a reservation for continuoustransmission (step S19). If the control section determines that there isno reservation (step S19; No), it sets I/B to be transmitted in the nextdownstream frame to I (the next frame is available) (step S20). If thecontrol section 101 determines that there is a reservation (step S19;Yes), it checks the value of the reservation counter 115 (step S21). Ifthe value of the reservation counter 115 is “1”, the control section 101cancels the reservation (by setting the count value of the reservationcounter 115 to 0) since the reserved data transmission is completed byupstream transmission in the next frame (step S22). If the value of thereservation counter 115 is not “1”, the control section 101 decrementsthe count value of the reservation counter 115 by 1 (step S23).

After processing in step S22 or S23, the control section 101 sets I/B tobe transmitted in the next downstream frame to B (transmissioninhibition) (step S24).

After the beginning of the next frame period, the control section 101transmits through the transmitting section 104 the collision controlinformation including R/N and PE set in step S12 or 13 and I/B set instep S15 or S24 (step S25) and further transmits the data portion in thenext frame (step S26).

The control section 101 repeatingly executes the above-describedoperation.

On the other hand, the control section 201 in the mobile station 12starts random access control processing shown in FIG. 6, for example, inresponse to a timer interrupt or the like during the period when theoperating mode is the receiving mode.

The control section 201 first determines whether or not there is anydata to be transmitted (step S31). Data to be transmitted is stored inadvance in the storage section 202 by being divided into packets, eachof which is a unit to be transmitted in one frame. Each packet includesheader information and a payload. The payload includes actual data to betransmitted and CRC information.

If the control section 201 determines that there is data to betransmitted (step S31; Yes), it sets 0 in the recycle counter 216 andsets a predetermined value in the reservation wait counter 215 (stepS32). Subsequently, the control section 201 activates the transmissionstart wait timer 213 and makes this timer start measuring a lapse oftime (step S33).

Subsequently, the control section 201 determines whether I/B incollision control information received in the immediately precedingdownstream frame is I (available) or B (transmission non-permission) bymeans of the header information analysis section 212 (step S34).

If the control section 201 determines that I/B=I, that is, the nextupstream frame is available (empty frame), it makes the datatransmitting/receiving section 211 transmit through the transmittingsection 204 the packet data corresponding to the first one frame in thedata not yet transmitted in the data to be transmitted (step S35).Further, the control section 201 increments the recycle counter 216 by 1(step S35).

Subsequently, the control section 201 changes the transmitting/receivingmode to the receiving mode, awaits reception of the next downstreamframe and receives the next downstream frame (step S36).

If the control section 201 determines that I/B=B in step S34, itdetermines whether or not the count value of the transmission start waittimer 213 activated in step S33 has reached a set value, that is, apredetermined time period has elapsed (step S37). If the predeterminedtime period has not elapsed (step S37; No), the control section 201changes the transmitting/receiving mode to the receiving mode, awaitsreception of the next downstream frame, receives the next downstreamframe (step S38), and returns to step S34.

If the control section 201 determines in step S37 that the predeterminedtime period has elapsed (step S37; Yes), it recognizes transmissionfailure and executes suitable processing (step S39).

When the control section 201 receives the next downstream frame throughthe receiving section 203 in step S36, it determines whether R/N in thereceived collision control information is R (the base station 11 hasreceived data) or N (the base station 11 has not received data) by meansof the header information analysis section 212 (step S40).

If R/N=R, that is, the base station 11 has received some data in thepreceding upstream frame (step S40; R), the control section 201determines whether or not PE contained in the received frame and the CRCcomputed value of the leading frame data transmitted in the precedingupstream frame in step S35 coincide with each other (step S41).

The determination result “Received PE=Transmitted CRC computed value”(step S41; Yes) means that the base station 11 has normally received theleading frame transmitted in step S35 (R/N=R) and has transmitted thecorresponding response. That is, the destination of the data in thedownstream frame presently received is this mobile station.

Subsequently, the control section 201 determines whether or not there isany remaining data to be transmitted (step S42). If there is no data tobe transmitted (step S42; No), the transmission process is completed.

If there is some remaining data to be transmitted (step S42; Yes), thecontrol section 201 determines whether P/D contained in the receivedframe is P (continuous transmission permission) or D (continuoustransmission non-permission) (step S43). If the determination result isP, the control section 201 changes the operating mode to thetransmitting mode and transmits the next frame of data (step S44). Thecontrol section 201 thereafter determines whether or not there is anyremaining data to be transmitted (step S45). If there is no data to betransmitted (step S45; No), the transmission process is completed.

If there is some remaining data to be transmitted, the control section201 returns to step S44 and successively transmits data in the nextupstream frame. In this case, the transmitting mode is maintained as theoperating mode of the mobile station 12.

If the control section 201 determines in step S43 that P/D=D, itreceives the next downstream frame by maintaining the receiving mode(step S46) without transmitting any data in the next upstream frame, anddetermines whether I/B in collision control information in the receivedframe is I (available) or B (transmission non-permission) (step S47). Ifthe control section 201 determines that I/B=I, that is, the nextupstream time is available, it returns to the above-described step S35to transmit the data in the leading frame.

If the control section 201 determines in step S47 that I/B=B, itdetermines whether or not the number of times processing results infailure to transmit data has reached the number of times set in advance,i.e., the number of times set in the reservation wait counter 215 (stepS48). If the set number of times has not been reached (step S48; Yes),the control section 201 decrements the value of the reservation waitcounter 215 by 1 (step S49) and returns to step S46. If the controlsection 201 determines that the number of times processing results infailure to transmit data has reached the number of times set in advance,i.e., the number of times set in the reservation wait counter 215 (stepS48; Yes), it recognizes transmission failure and executes predeterminedprocessing (step S50).

If the result of determination in step S40 is R/N=N, or if the result ofdetermination in step S41 is that the received PE does not coincide withthe transmitted CRC value, it indicates that the data in the leadingframe transmitted in step S35 has not been normally received by the basestation 11 for some reason. That is, R/N=N indicates that the entiredata has not been received, and the result that the received PE does notcoincide with the transmitted CRC value indicates that data from one ofthe other mobile stations has been received or the data transmitted instep S35 has been received while some of the contents of data is beingerroneously recognized. In this case, the control section 201 determineswhether or not the count value of the recycle counter 216, i.e., thenumber of times the leading frame is transmitted (recycle time), hasreached the number of times set in advance (step S51).

If the number of times the leading frame is transmitted has reached thenumber of times set in advance (step S51; Yes), the control section 201recognizes transmission failure and performs the predeterminedprocessing (step S39).

If the count value of the recycle counter 216 has not reached the setvalue, the control section 201 randomly determines a delay time bygenerating a random number by means of the random delay section 214 inorder to again transmit the leading frame, measures the delay time (stepS52) and thereafter returns to step S33.

The above-described configurations of the base station 11 and the mobilestations 12 and the above-described random access operation performedbetween the base station 11 and the mobile stations 12 ensure that evenin FDMA in which transmission and reception are continuously performedwith respect to time, each mobile station 12 can refer to P/Dinformation indicating whether or not continuous transmission ispermitted and can be on standby for transmission. Therefore a sufficienttime for control in the base station 11 can be provided and the controlprocess can be simplified.

The mobile station 12 checks whether or not transmission of atransmitted leading frame (or a single frame before a continuoustransmission permission is given) has been accepted by the base station11. If the mobile station 12 determines that the transmission has beenaccepted, it transmits subsequent data. Therefore, no useless collisionoccurs between the plurality of mobile stations 12.

Even during the time period for two frames immediately after receivingthe leading frame from one mobile station 12, all the mobile stations 12can transmit data to the base station 11. Therefore the throughput ishigh.

It is possible to control, with respect to times including the switchingtime, even the mobile station 12 arranged as a half-duplex type or thelike incapable of simultaneously performing transmission and reception.

The essentials of various conditions for execution of theabove-described random access control are collectively described below.

-   1) Preconditions

1-1) A slotted-ALOHA random access control method for mobilecommunication is used.

1-2) Base station 11 is FDM, while each mobile station is FDMA.

1-3) A transmitting-receiving offset time is set between transmissionand reception in the frame structure formed by base station 11 andmobile station 12.

1-4) Collision control information is placed in a frame of downstreamcontrol data transmitted from the base station.

1-5) The base station notifies collision control information to allthe-mobile stations.

1-6) Collision control information includes “availability/inhibition”information (I/B) as to whether or not an upstream frame is available ortransmission-inhibited.

1-7) Collision control information includes “reception/non-reception”information (R/N) indicating whether or not an upstream signal has beenreceived by the base station 11.

1-8) Collision control information includes “received data” information(PE) indicating to which mobile station information is destined.

1-9) The mobile station can determine whether or not data transmittedfrom itself has been normally received by the base station, by analyzing“reception/non-reception” information and “received data” information incollision control information.

1-10) Data transmitted from mobile station 12 includes data indicatingthe number of frames (e.g., the total number of frame and the number ofremaining frames) before the completion of transmission of the entireinformation.

1-11) “Received data” information is the results of CRC determinationmade in the base station on frame data transmitted from mobile stations.

-   2) Conditions in this embodiment

2-1) Collision control information includes “continuous transmissionpermission/non-permission” information indicating whether or notcontinuous transmission of upstream frames is permitted.

2-2) “Availability/inhibition” information in collision controlinformation is information on an upstream frame having the same numberas that of the frame through which the collision control information isnotified.

2-3) “Reception/non-reception” information in collision controlinformation is information on an upstream signal having the thirdpreceding frame number with respect to the frame number through whichthe information is notified.

2-4) “Continuous transmission permission/non-permission” information incollision control information is information on an upstream frame havingthe same frame number as that of the frame through which the informationis transmitted.

2-5) “Received data” information in collision control information isinformation on an upstream signal having the third preceding framenumber with respect to the frame number through which the collisioncontrol information is notified.

2-6) Mobile station 12 can transmit a single frame (packet) when“availability/inhibition” information designates availability.

2-7) If mobile station 12 determines that a single frame transmittedfrom itself has been received by the base station, and if “continuoustransmission permission/non-permission” information designatescontinuous transmission permission, mobile station 12 can continuouslytransmit frames from the next transmission.

2-8) If mobile station 12 determines that a single frame transmittedfrom itself has been received by the base station, and if “continuoustransmission permission/non-permission” information designatescontinuous non-permission, mobile station 12 can activate the“reservation wait counter” changed on a frame-by-frame basis and can beon standby for transmission before the completion of the operation ofthe “reservation wait counter”.

2-9) After the completion of the operation of the “reservation waitcounter”, random access failure is recognized and random access isretried.

The above-described operations will be described on the basis of aconcrete example.

An example of the operations in a case where one mobile stationtransmits one frame of data without collision with other mobile stationswill first be described with reference to FIG. 7.

Alphabetic letters showing in downstream frames respectively indicateddenotations of I/B, R/N, P/D and PE. For example, “I, N, D, 0” enteredtherein means that I/B=I (an upstream time is available); R/N=N (anupstream signal having the third preceding frame number has not beenreceived); P/D=D (continuous transmission from the next upstream time isnot permitted); and the content of PE is 0.

In the example shown in FIG. 7, I/B contained in collision controlinformation received through a downstream frame #0 is I, that is, theupstream frame #0 is available. The control section 201 transmitsthrough the transmitting section 204 information to be transmitted, aswell as a CRC result (A). At this time, header information includes thetotal number of groups of data (the number of slots: 1 in this case).

The base station 11 receives through the antenna 105 the datatransmitted from the mobile station 12, restores the data bydemodulation in the receiving section 103, and supplies the data to thecontrol section 101.

The control section 101 analyzes the received data and confirms whetherthe reception has been completed without error, for example, byperforming a CRC check. If the reception has been correctly performed,the control section 101 sets R/N in the collision control information toR, sets the result of CRC computation on the received data as PE, andtransmits a third downstream frame #3 by containing these informationitems.

The mobile station 12 receives the downstream frame #3, demodulates andanalyzes the received frame and determines from R/N whether or not thebase station 11 has succeeded in reception. In the example shown in FIG.7, the control section 201 determines that the base station 11 hasreceived the data, since R/N is R. Further, the control section 201determines that the base station 11 has correctly received the data fromthe information indicating that PE is A and that the CRC on thetransmitted data is also A, and thereby knows that the transmission hasended in success.

An example of the operations in a case where mobile station 12 transmitsone frame of data but a failure to achieve transmission to the basestation 12 results due to a transmitted data collision with anothermobile station 12 and a certain wireless condition will next bedescribed with reference to FIG. 8.

A situation will be considered in which a mobile station MA receives I/Bat a time T1; the received I/B is I; the mobile station MA thereforestarts transmission; a mobile station MB also starts transmissionsimultaneously; collision occurs between the mobile stations; and datafrom the mobile station MB reaches the base station 11.

The base station 11 sets R/N to R, sets PE to “B”, which is the same asthe CRC computed value transmitted from the mobile station B, andnotifies (transmits) the information at time T2 three frames after.

The mobile stations MA and MB receive collision information at time T2.The mobile station MA determines transmission failure, while the mobilestation MB determines transmission success.

Because of transmission failure, the mobile station MA sets a randomdelay, performs retransmission at time T3, and also ended in failure toachieve transmission to the base station 11 due to a bad wirelesscondition. Accordingly, the base station 11 notifies information at timeT4 by setting RIN to N and PE to 0. The mobile station MA thereby knowstransmission failure, again performs retransmission at time T5 after arandom delay, and succeeds in transmission at time T6 at which R/Nbecomes R and CRC in PE becomes “A” as a matching result, thuscompleting random access.

An example of collision control in a case where mobile stations MA andMB transmit four consecutive groups of data will next be described withreference to FIG. 9.

It is assumed that, in this example, when the base station 11 transmitsdata to a particular mobile station 12, information indicating themobile station 12 as a destination is included in the transmitted data.

The base station 11 first sets R/N=N and PE=0 in 0th to second frames #0to #2 since there is no reception event three frames before (step S13),and also sets P/D=P by determining that there is no reservation (stepS17).

The mobile station MA has data to be transmitted (step S31 in FIG. 6;Yes), and I/B in the received 0th frame #0 is I (step S34; I). Themobile station MA therefore transmits a leading-frame packet in datapackets corresponding to four frames not yet transmitted (step S35).This packet includes, as information in header information, informationindicating that the sender is MA, information indicating that the packetis a leading frame, and information indicating that total informationlength is 4 and there are three remaining frames. Also, 1₁ is added asCRC information to the header information.

The base station 11 receives and analyzes the data transmitted from themobile station MA. The base station 11 obtains information including theinformation indicating that the transmitted data is a leading frame andthe information indicating that there are three remaining frames ofdata, and performs a CRC check (it is assumed that correctness isconfirmed from CRC=1₁ in this example).

The base station receives the data, confirms correctness by the CRCcheck, therefore determines Yes in step S11 in FIG. 5, and sets R/N inthe third frame #3 to R and sets the CRC value 11 in PE (step S12).Since the received data is a leading frame and since remaining framesexist, the result of determination in step S14 is Yes and determinationis made in step S16 as to whether or not there is a reservation. In thisexample, it is determined that there is no reservation (step S16; No),P/D in the third frame is set to P (step S17) and a reservationcondition and the number of remaining frames “3” are set in thereservation counter 115 (step S18).

Determination is thereafter made in step S19 as to whether or not thereis a reservation. Since a reservation is set in step S18, the result ofdetermination in step S19 is Yes and the count value of the reservationcounter 115 is updated from 3 to 2 (step S23) and I/B=B is set (stepS24). Subsequently, the data in the third frame #3 is transmitted (stepsS25 and S26). The header information in the third frame #3 includesinformation for identification of the mobile station MA as adestination.

At a time corresponding to the fourth downstream frame #4, the countvalue of the reservation counter 115 of the base station 11 is updatedfrom 2 to 1 (step S23), and the reservation is reset with respect to thefifth frame #5 (step S22). Also in the fourth and fifth frames #4 and#5, I/B=B (step S24).

The mobile station MA receives the third downstream frame #3 addressedto itself (step S36). R/N is R (step S40; R), PE coincides with CRC=1₁that the mobile station MA has transmitted (step S41; Yes), and P/D is P(step S43; P). The mobile station MA therefore transmits the remainingthree frames of data in the consecutive third, fourth and fifth upstreamframes #3, #4, and #5 (steps S44 and S45). By completing transmission ofthe entire data, the mobile station MA stops transmitting.

A mobile station MC has one frame of data to be transmitted andtransmits the data since I/B in the first frame #1 is I.

The base station 11 receives and analyzes the data transmitted from themobile station MC. The base station 11 obtains information includinginformation indicating that the sender is MC, information indicatingthat the transmitted data is a leading frame, and information indicatingthat there is no remaining frame of data, and performs a CRC check (itis assumed that correctness is confirmed from CRC=31 in this example).

The base station 11 confirms correctness of the received data by the CRCcheck, therefore determines Yes in step S11 in FIG. 5, and sets R/N inthe fourth frame #4 to R and sets the CRC value 3₁ in PE (step S12).

The received data is a leading frame but no remaining frames exist.Accordingly, the result of determination in step S14 is No and P/D inthe fourth frame is set to D in step S15. The reservation counter 115 isin a reservation condition with respect to the third frame (step S19;Yes). The count value of the reservation counter 115 is updated from 2to 1. (step S23). I/B=B is set (step S24). The fourth frame #4 istransmitted to the mobile station MC (steps S25 and S26).

The mobile station MC receives the fourth downstream frame #4 from thebase station 11 (step S36). It is determined that R/N is R and PE=31(step S40; R, step S41; Yes). There is no remaining data (step S42; No)and the transmission is completed.

The mobile station MB also has data to be transmitted (step S31 in FIG.6; Yes), and I/B in the received second frame #2 is I (step S34; I). Themobile station MB therefore transmits a leading-frame packet (step S35).This packet includes, in the header, information indicating that thepacket is a leading frame, and information indicating that totalinformation length is 4 and there are three remaining frames. Also, 2₁is added as CRC information.

The base station 11 receives and analyzes the data transmitted from themobile station MB. The base station 11 obtains information including theinformation indicating that the transmitted data is a leading frame andthe information indicating that there are three remaining frames ofdata, and performs a CRC check (it is assumed that correctness isconfirmed from CRC=2₁ in this example).

The base station 11 receives the data, confirms correctness by the CRCcheck, therefore determines Yes in step S11 in FIG. 5, and sets R/N inthe fifth frame #5 to R and sets the CRC value 2₁ in PE (step S12).Since the received data is a leading frame and since remaining framesexist, the result of determination in step S14 is Yes and determinationis made in step S16 as to whether or not there is a reservation. In thisexample, there is a reservation (step S16; Yes), P/D in the fifth frameis set to D (step S15). Determination is subsequently made as to whethera reservation condition is set in the reservation counter 115 (stepS19). In this example, there is a reservation (step S19; Yes), and thevalue of the reservation counter 115 is “1” (step S21; =1). Accordingly,the reservation is canceled (step S22), I/B=B is set (step S24) and thefifth frame is transmitted to the mobile station MB.

The mobile station MB receives the fifth downstream frame #5 (step S36).R/N is R (step S40; R), PE coincides with CRC=2₁ that the mobile stationMB has transmitted (step S41; Yes). There are remaining frames (stepS42; Yes). P/D is therefore determined (step S43). Since P/D is D, thenext frame, i.e., the sixth frame #6, is received in step S46.

I/B in the sixth frame #6 is IB=I. Accordingly, the process returns tostep S35, the data in the leading frame in the remaining data istransmitted, and the value of the recycle counter is incremented by 1.In this example, since one frame of data has been transmitted in thesecond frame #2, the data in the second frame, i.e., the leading framein the remaining three frames of data, is transmitted.

The base station 11 receives the data transmitted from the mobilestation MB, obtains information including the information indicatingthat the transmitted data is a leading frame and the informationindicating that there are three remaining frames of data, and performs aCRC check (it is assumed that correctness is confirmed from CRC=2₂ inthis example).

The base station determines Yes in step S11 in FIG. 5, and sets R/N inthe ninth frame #9 to R and sets the CRC value 22 in PE (step S12).Further, Yes is determined in step S14, it is determined in step S16that there is no reservation (step S16; No), and P/D is set to P (stepS17). Subsequently, the number of remaining frames “2” is set in thereservation counter 115 (step S18).

Determination is thereafter made in step S19 as to whether or not thereis a reservation. Since a reservation is set in step S18, the result ofdetermination in step S19 is Yes and the count value of the reservationcounter 115 is updated from 2 to 1 (step S23). I/B=B is set (step S24)and the data in the third frame #3 is transmitted (steps S25 and S26).

The mobile station MB receives the ninth frame #9 from the base station11 in step S36, R/N=R, PE=2₂ as a matching result, and the received P/Dis P (step S43; P). The mobile station MB therefore transmits the ninthand tenth frames #9 and #10 (steps S44 and S45).

Even if the data in the fourth frame in the data to be transmitted isnot normally received for some reason, the present data transmission bythe mobile station MB can be terminated, as shown in the figure.

The present invention is not limited to the above-described embodiment.Various modifications and applications are possible. For example, whilethe embodiment has been described with respect to the case where themobile stations 12 are of a half-duplex type, random access control inaccordance with the present invention can also be applied, without beingmodified, to a network in which only full-duplex mobile stations or bothfull-duplex and half-duplex mobile stations exist.

While a 1.5-frame period is mentioned above as the delay time by whichupstream frames are delayed with respect to downstream frames, the delaytime may be set to any time period longer than the one-frame periodaccording to the processing capacity of the mobile station 12. Forexample, it may be set to a 1.2 frame period, a 2.5-frame period, a3.5-frame period or a 3-frame period. The base station 11 setsinformation with respect to each of frame periods for downstream framesas required according to this frame delay setting.

While the embodiment has been described with respect to a form based onthe ICMA-PE method, it is not necessary to definitely specify each ofthe collision control information items I/B, R/N, and P/D. For example,different states such as a “state 1” in which all mobile stations canperform transmission, a “state 2” in which a particular one of themobile stations is permitted to perform continuous transmission, and a“state 3” in which a particular one of the mobile stations iscontinuously performing transmission may be defined.

In the embodiment, in a case where the amount of communication trafficis large and where the base station 11 should disperse continuoustransmission by mobile stations 12 with respect to time, the basestation 11 can disperse continuous transmission by the mobile stations12 with respect to time by designating D as P/D and by setting the“reservation wait counter” to a suitable value according to a directionfrom the base station 11.

For example, the base station 11 may perform control by setting P/D to Dwhen the base station 11 should inhibit continuous transmission from themobile stations 12 in a situation where the amount of traffic is large,and may perform control by unexceptionally setting P/D to P ifcontinuous transmission can be permitted in a situation where the amountof traffic is small.

For example, referring to FIG. 10, traffic is measured (step S61). Whenthe measured amount of traffic is larger than a second reference amount(step S61; Yes), the proportion of D in P/D is set to K1% and theinitial value of the reservation wait counter is set to K2 (step S63).

When the measured amount of traffic is within the range from a firstreference amount to the second reference amount (step S62; No, step S64;Yes), the proportion of D in P/D is set to M1% and the initial value ofthe reservation wait counter is set to M2 (K1>M1, K2>M2; step S65). Whenthe amount of traffic is smaller than the first reference amount (stepsS62 and S64; No), the traffic may be left without being controlled.

A program for enabling ordinary base station and mobile stations toperform the above-described random access control may be prepared andinstalled in the ordinary base station and mobile stations to execute.

1. A mobile communication system in which a base station and mobilestations perform communication by a slotted-ALOHA method, the systembeing characterized in that a predetermined offset time is set betweendownstream communication frames from the base station to each mobilestation and upstream communication frames from the mobile station to thebase station; the base station transmits a transmission permissionsignal for permitting transmission of one frame of data when aparticular one of the upstream communication frames is available,determines whether or not continuous transmission of subsequent dataover a plurality of frames should be permitted if the subsequent dataexists subsequently to the one frame of data received through theparticular frame from one of the mobile stations, and transmits acontinuous transmission permission signal for permission of thecontinuous transmission when the continuous transmission is permitted;and if each mobile station has data to be transmitted, it transmits oneframe of data in response to the transmission permission signal from thebase station, and transmits the subsequent data through a plurality ofconsecutive frames in the upstream communication frames when receivingthe continuous transmission permission signal with respect to thetransmitted one frame of data.
 2. The mobile communication systemaccording to claim 1, characterized in that the upstream communicationframes from the mobile station to the base station are delayed from thedownstream communication frames from the base station to the mobilestation by a predetermined time period longer than one frame; the basestation transmits the transmission permission signal through the framein the downstream communication frames corresponding to the first framein the upstream communication frames if the first frame in the upstreamcommunication frames is available; if the mobile station has data to betransmitted, it transmits the one frame of data through the first framein the upstream communication frames when receiving the transmissionpermission signal through the frame corresponding to the first frame;determination is made as to whether or not continuous transmission ofsubsequent data over a plurality of frames should be permitted if thesubsequent data exists subsequently to the one frame of data receivedthrough the particular frame from the mobile station, and the continuoustransmission permission signal is transmitted through the second frame apredetermined number of frames after the frame corresponding to thefirst frame when the continuous transmission is permitted; and when themobile station receives the continuous transmission permission signalthrough the second frame in the downstream communication frames, ittransmits the subsequent data through a plurality of consecutive framesin the upstream communication frames the leading one of whichcorresponds to the second frame.
 3. The mobile communication systemaccording to claim 1, characterized in that when the mobile stationreceives the continuous transmission permission signal, it transmits thesubsequent data through the consecutive frames without checking whetheror not the base station has succeeded in reception.
 4. The mobilecommunication system according to claim 1, characterized in that themobile station has a half-duplex-type configuration capable ofselectively executing transmission processing and reception processing;and when the mobile station receives the transmission permission signalin a receiving mode, and if data to be transmitted exists, it transmitsthe one frame of data in the upstream communication frame by selecting atransmitting mode in place of the receiving mode, thereafter receivesthe downstream communication frame by selecting the receiving mode, and,when receiving the continuous transmission permission signal,continuously transmits the subsequent data through the plurality offrames in the upstream communication frames by selecting thetransmitting mode.
 5. The mobile communication system according to claim1, characterized in that the base station transmits, together with thecontinuous transmission permission signal, information as to whether ornot one frame of data has been normally received from the mobilestation; and the mobile station determines whether or not the basestation has normally received the one frame of data transmitted from themobile station, and transmits the subsequent data if it determines thatthe base station normally receives the data.
 6. The mobile communicationsystem according to claim 1, characterized in that the base stationtransmits mobile station identification information for identificationof one of the mobile stations together with the continuous transmissionpermission signal; and the mobile station transmits the subsequent datawhen the mobile station identification information designates the mobilestation.
 7. The mobile communication system according to claim 1,characterized in that the mobile station transmits information foridentification of the number of frames of the subsequent data togetherwith the one frame of data; the base station transmits a transmissioninhibition signal for inhibiting data transmission from the other mobilestations during transmission of the subsequent data by the mobilestation on the basis of the number of frames of the subsequent datanotified from the mobile station; and the mobile station that hastransmitted the one frame of data in the mobile stations continuouslytransmits the subsequent data according to the continuous transmissionpermission signal, the other mobile stations restraining themselves inresponse to the transmission inhibition signal from performing datatransmission during transmission of the subsequent data.
 8. The mobilecommunication system according to claim 1, characterized in that if themobile station cannot receive the transmission permission signal duringa predetermined time period in a case where it has data to betransmitted, it recognizes transmission failure, sets a delay time, andagain executes processing for awaiting reception of the transmissionpermission signal after a lapse of time through the delay time.
 9. Themobile communication system according to claim 1, characterized in thatthe base station transmits a continuous transmission non-permissionsignal when it does not permit continuous transmission of datasubsequent to the one frame of data; and each mobile station determineswhether or not the base station has received the one frame of datatransmitted from the mobile station, keeps on standby for transmissionof the subsequent data after a lapse of a predetermined frame period ifit determines that the base station has received the data, and if it hasreceived the continuous transmission non-permission signal, andtransmits the leading one frame of data in the subsequent data if itreceives the transmission permission signal during standby.
 10. Themobile communication system according to claim 1, characterized in thatthe base station includes means of determining whether or not thecontinuous transmission should be permitted on the basis of a conditionof traffic.
 11. A base station for use in a mobile communication systemin which communication is performed by a slotted-ALOHA method, the basestation being characterized in that a predetermined offset time is setbetween downstream communication frames from the base station to amobile station and upstream communication frames from the mobile stationto the base station, the base station also being characterized bycomprising: transmission permission signal transmitting means oftransmitting, through a predetermined frame in the downstreamcommunication frames, a transmission permission signal for permittingtransmission of one frame of data when one of the upstream communicationframes is available; receiving means of receiving one frame of datatransmitted from the mobile station through one of the upstreamcommunication frames corresponding to the predetermined frame; andcontinuous transmission permission signal transmitting means ofdetermining whether or not continuous transmission of subsequent two ormore frames of data should be permitted if the subsequent frames of dataexists subsequently to the received one frame of data, and transmittingthrough the downstream communication frame a continuous transmissionpermission signal for permission of the continuous transmission when thecontinuous transmission is permitted.
 12. A mobile station for use in amobile communication system in which communication is performed by aslotted-ALOHA method, the mobile station being characterized-in that apredetermined offset time is set between downstream communication framesfrom a base station to the mobile station and upstream communicationframes from the mobile station to the base station, the mobile stationalso being characterized by comprising: transmission permission signalreceiving means of receiving a transmission permission signal from thebase station; leading data transmitting means of transmitting one frameof leading data through one of the upstream communication frames inresponse to the transmission permission signal if the data to betransmitted exists; continuous transmission permission signal receivingmeans of receiving one of the downstream communication framessubsequently to transmission of the leading data to receive a continuoustransmission permission signal; and continuous transmission means ofcontinuously transmitting data subsequent to the leading data through aplurality of frames in the upstream communication frames in response tothe continuous transmission permission signal.
 13. A communicationmethod in which a first and second communication devices performcommunication by a slotted-ALOHA method, the method being characterizedin that a predetermined offset time is set between first communicationframes from the first communication device to the second communicationdevice and second communication frames from the second communicationdevice to the first communication device, and a transmission permissionsignal for permitting transmission of one frame of data when aparticular one of the second communication frames is available istransmitted from the first communication device to the secondcommunication device; one frame of data in three or more frames of datato be transmitted is transmitted from the second communication device tothe first communication device in response to the transmissionpermission signal; determination is made on the basis of the one frameof data as to whether or not continuous transmission of data subsequentto the one frame of data over a plurality of frames should be permitted;a continuous transmission permission signal for permitting thecontinuous transmission is transmitted from the first communicationdevice to the second communication device when the continuoustransmission is permitted; and the subsequent data is transmittedthrough a plurality of consecutive frames in the second communicationframes in response to the continuous transmission permission signal. 14.A medium on which a computer program is recorded, the computer programenabling a computer having a communication function to function as abase station for use in a mobile communication system in which an offsettime is set between downstream communication frames from the basestation to a mobile station and upstream communication frames from themobile station to the base station, and in which communication isperformed by a slotted-ALOHA method, the base station having:transmission permission signal transmitting means of transmitting,through a predetermined frame in the downstream communication frames, atransmission permission signal for permitting transmission of one frameof data when one of the upstream communication frames is available;receiving means of receiving one frame of data transmitted from themobile station through one of the upstream communication framescorresponding to the predetermined frame; and continuous transmissionpermission signal transmitting means of determining whether or notcontinuous transmission of subsequent two or more frames of data shouldbe permitted if the subsequent frames of data exists subsequently to thereceived one frame of data, and transmitting through the downstreamcommunication frame a continuous transmission permission signal forpermission of the continuous transmission when the continuoustransmission is permitted.
 15. A medium on which a computer program isrecorded, the computer program enabling a computer having acommunication function to function as a mobile station for use in amobile communication system in which an offset time is set betweendownstream communication frames from a base station to the mobilestation and upstream communication frames from the mobile station to thebase station, and in which communication is performed by a slotted-ALOHAmethod, the mobile station having: transmission permission signalreceiving means of receiving a transmission permission signal from thebase station; leading data transmitting means of transmitting one frameof leading data through one of the upstream communication frames inresponse to the transmission permission signal if the data to betransmitted exists; continuous transmission permission signal receivingmeans of receiving one of the downstream communication framessubsequently to transmission of the leading data to receive a continuoustransmission permission signal; and continuous transmission means ofcontinuously transmitting data subsequent to the leading data through aplurality of frames in the upstream communication frames in response tothe continuous transmission permission signal.